The present invention relates to improvements in a slip control system of a torque converter for an automatic transmission, and more particularly to a slip control system which quickly converges a slip rotation speed between input and output elements of a torque converter to a target value according to a vehicle running condition.
Generally, a torque converter has merits such as a torque fluctuation absorbing function and a torque increasing function due to a fluid power transmission, although a transmission efficiency of the fluid power transmission is lower than that of a power transmission by means of a direct mechanical connection. In order to improve the transmission efficiency of the torque converter, most torque converters employ lockup clutches for directly connecting the input and output elements of the torque converter when the vehicle is put in a running condition where the torque fluctuation absorbing function or the torque increasing function are not required. However, such a torque converter with a lockup clutch, which is merely controlled between the lockup state (ON) and the converter state (OFF), has a limitation in a slip control range, and therefore it is difficult to further improve the transmission efficiency of a torque converter with this ON-OFF control type lockup clutch.
In order to solve the above-mentioned problems of the conventional ON-OFF lockup control type torque converter, various types of lockup torque converters have been proposed. For example, there has been proposed a lockup torque converter which is arranged to set a slip control range upon ensuring the necessary and minimum torque fluctuation absorbing function and torque increasing function. Further, an art for controlling slippage of a torque converter according to a target slip rotation speed determined by a vehicle running condition is disclosed in a paper "Luck-up Clutch Slip Control System for Automatic Transmissions", which is written on pages 81 to 84 in a preprint for a scientific lecture meeting No. 954 in 1995-9 published by a corporation of Japanese Society of Automotive Engineering. More particularly, in this art a target slip rotation speed is obtained from the throttle opening, a vehicle speed, and an oil temperature of the automatic transmission as an input. A lockup clutch engagement pressure command value (required engagement force) for achieving the target slip rotation speed is directly obtained by means of the parameter identification method where the target slip rotation speed and an actual slip rotation speed are treated as an input and an output, in order to output a control signal corresponding to the engagement pressure command value.